Effects of uncertainties on seismic behaviour of optimum designed braced steel frames

Iman Hajirasouliha,Kypros Pilakoutas,Reza K. Mohammadi 국제구조공학회 2016 Steel and Composite Structures, An International J Vol.20 No.2

Concentrically braced steel frames (CBFs) can be optimised during the seismic design process by using lateral loading distributions derived from the concept of uniform damage distribution. However, it is not known how such structures are affected by uncertainties. This study aims to quantify and manage the effects of structural and ground-motion uncertainty on the seismic performance of optimum and conventionally designed CBFs. Extensive nonlinear dynamic analyses are performed on 5, 10 and 15-storey frames to investigate the effects of storey shearstrength and damping ratio uncertainties by using the Monte Carlo simulation method. For typical uncertainties in conventional steel frames, optimum design frames always exhibit considerably less inter-storey drift and cumulative damage compared to frames designed based on IBC-2012. However, it is noted that optimum structures are in general more sensitive to the random variation of storey shear-strength. It is shown that up to 50% variation in damping ratio does not affect the seismic performance of the optimum design frames compared to their code-based counterparts. Finally, the results indicate that the ground-motion uncertainty can be efficiently managed by optimizing CBFs based on the average of a set of synthetic earthquakes representing a design spectrum. Compared to code-based design structures, CBFs designed with the proposed average patterns exhibit up to 54% less maximum inter-storey drift and 73% less cumulative damage under design earthquakes. It is concluded that the optimisation procedure presented is reliable and should improve the seismic performance of CBFs.

Compressive behaviour of circular steel tube-confined concrete stub columns with active and passive confinement

Mahdi Nematzadeh,Iman Hajirasouliha,Akbar Haghinejad,Morteza Naghipour 국제구조공학회 2017 Steel and Composite Structures, An International J Vol.24 No.3

This paper presents the results of a comprehensive experimental investigation on the compressive behaviour of steel tube-confined concrete (STCC) stub columns with active and passive confinement. To create active confinement in STCC columns, an innovative technique is used in which steel tube is laterally pre-tensioned while the concrete core is simultaneously pre-compressed by applying pressure on fresh concrete. A total of 135 STCC specimens with active and passive confinement are tested under axial compression load and their compressive strength, ultimate strain capacity, axial and lateral stress.strain curves and failure mode are evaluated. The test variables include concrete compressive strength, outer diameter to wall thickness ratio of steel tube and prestressing level. It is shown that applying active confinement on STCC specimens can considerably improve their mechanical properties. However, applying higher prestressing levels and keeping the applied pressure for a long time do not considerably affect the mechanical properties of actively confined specimens. Based on the results of this study, new empirical equations are proposed to estimate the axial strength and ultimate strain capacity of STCC stub columns with active and passive confinement.

Seismic response modification factors for stiffness degrading soil-structure systems

Ganjavi, Behnoud,Bararnia, Majid,Hajirasouliha, Iman 국제구조공학회 2018 Structural Engineering and Mechanics, An Int'l Jou Vol.68 No.2

This paper aims to develop response modification factors for stiffness degrading structures by incorporating soil-structure interaction effects. A comprehensive parametric study is conducted to investigate the effects of key SSI parameters, natural period of vibration, ductility demand and hysteretic behavior on the response modification factor of soil-structure systems. The nonlinear dynamic response of 6300 soil-structure systems are studied under two ensembles of accelograms including 20 recorded and 7 synthetic ground motions. It is concluded that neglecting the stiffness degradation of structures can results in up to 22% underestimation of inelastic strength demands in soil-structure systems, leading to an unexpected high level of ductility demand in the structures located on soft soil. Nonlinear regression analyses are then performed to derive a simplified expression for estimating ductility-dependent response modification factors for stiffness degrading soil-structure systems. The adequacy of the proposed expression is investigated through sensitivity analyses on nonlinear soil-structure systems under seven synthetic spectrum compatible earthquake ground motions. A good agreement is observed between the results of the predicted and the target ductility demands, demonstrating the adequacy of the expression proposed in this study to estimate the inelastic demands of SSI systems with stiffness degrading structures. It is observed that the maximum differences between the target and average target ductility demands was 15%, which is considered acceptable for practical design purposes.

Numerical solution of beam equation using neural networks and evolutionary optimization tools

Babaei, Mehdi,Atasoy, Arman,Hajirasouliha, Iman,Mollaei, Somayeh,Jalilkhani, Maysam Techno-Press 2022 Advances in computational design Vol.7 No.1

In this study, a new strategy is presented to transmit the fundamental elastic beam problem into the modern optimization platform and solve it by using artificial intelligence (AI) tools. As a practical example, deflection of Euler-Bernoulli beam is mathematically formulated by 2nd-order ordinary differential equations (ODEs) in accordance to the classical beam theory. This fundamental engineer problem is then transmitted from classic formulation to its artificial-intelligence presentation where the behavior of the beam is simulated by using neural networks (NNs). The supervised training strategy is employed in the developed NNs implemented in the heuristic optimization algorithms as the fitness function. Different evolutionary optimization tools such as genetic algorithm (GA) and particle swarm optimization (PSO) are used to solve this non-linear optimization problem. The step-by-step procedure of the proposed method is presented in the form of a practical flowchart. The results indicate that the proposed method of using AI toolsin solving beam ODEs can efficiently lead to accurate solutions with low computational costs, and should prove useful to solve more complex practical applications.

Structural Design Optimization of All-Steel Buckling-Restrained Braces Using Intelligent Optimizers

Seyed Mohamad Hoseini,Hossein Parastesh,Iman Hajirasouliha,Ahmad Ferdowsi 한국강구조학회 2021 International Journal of Steel Structures Vol.21 No.6

This study aims to introduce a novel optimal structural design framework for buckling-restrained braces (BRBs) in multi-story buildings. Five artifi cial intelligence (AI) algorithms, including particle swarm optimization (PSO), shuffl ed frog-leaping algorithm (SFLA), interior search algorithm (ISA), hybrid of bat and particle swarm optimization (BAT-PSO), and political optimizer, which is recently proposed, are adopted for the optimum design of BRB systems. In the proposed optimization process, the BRB cross-sectional area is taken as the objective function considering the stiff ness-strength criteria. As a result, by optimizing the BRB cross-sectional area, the weight of BRBs will reduce. Two mostly-used cross-sectional profi les for all-steel BRBs (circular and rectangular) are considered. In general, the optimized solutions using AI algorithms were more cost eff ective and exhibited considerably better structural performance in terms of global buckling requirements in comparison to other conventional BRB designs. The results showed that BAT-PSO worked the best in terms of objective function value and computational time. The design solutions obtained using BAT-PSO were lighter (35% for circular profi les and 20% for rectangular profi les), and had superior performance in terms of both stiff ness and strength in comparison with the conventional BRB designs. It was also shown that using circular profi le can reduce the weight of BRB elements by around 15% compared to rectangular profi le. The results of this study should prove useful in more effi cient design of BRB systems in common practice.

Experimental study and calculation of laterally-prestressed confined concrete columns

Mahdi Nematzadeh,Saeed Fazli,Iman Hajirasouliha 국제구조공학회 2017 Steel and Composite Structures, An International J Vol.23 No.5

In this paper, the effect of active confinement on the compressive behaviour of circular steel tube-confined concrete (STCC) and concrete-filled steel tube (CFST) columns is investigated. In STCC columns the axial load is only applied to the concrete core, while in CFST columns the load is carried by the whole composite section. A new method introduced to apply confining pressure on fresh concrete by laterally prestressing steel tubes. In order to achieve different prestressing levels, shortterm and long-term pressures are applied to the fresh concrete. Three groups of STCC and CFST specimens (passive, S-active and L-active groups) are tested under axial loads. The results including stress-strain relationships of composite column components, secant modulus of elasticity, and volumetric strain are presented and discussed. Based on the elastic-plastic theory, the behaviour of the steel tube is also analyzed during elastic, yielding, and strain hardening stages. The results show that using the proposed prestressing method can considerably improve the compressive behaviour of both STCC and CFST specimens, while increasing the prestressing level has insignificant effects. By applying prestressing, the linear range in the stress-strain curve of STCC specimens increases by almost twice as much, while the improvement is negligible in CFST specimens.

Seismic response modification factors for stiffness degrading soil-structure systems

Behnoud Ganjavi,Majid Bararnia,Iman Hajirasouliha 국제구조공학회 2018 Structural Engineering and Mechanics, An Int'l Jou Vol.68 No.2

This paper aims to develop response modification factors for stiffness degrading structures by incorporating soil-structure interaction effects. A comprehensive parametric study is conducted to investigate the effects of key SSI parameters, natural period of vibration, ductility demand and hysteretic behavior on the response modification factor of soil-structure systems. The nonlinear dynamic response of 6300 soil-structure systems are studied under two ensembles of accelograms including 20 recorded and 7 synthetic ground motions. It is concluded that neglecting the stiffness degradation of structures can results in up to 22% underestimation of inelastic strength demands in soil-structure systems, leading to an unexpected high level of ductility demand in the structures located on soft soil. Nonlinear regression analyses are then performed to derive a simplified expression for estimating ductility-dependent response modification factors for stiffness degrading soil-structure systems. The adequacy of the proposed expression is investigated through sensitivity analyses on nonlinear soil-structure systems under seven synthetic spectrum compatible earthquake ground motions. A good agreement is observed between the results of the predicted and the target ductility demands, demonstrating the adequacy of the expression proposed in this study to estimate the inelastic demands of SSI systems with stiffness degrading structures. It is observed that the maximum differences between the target and average target ductility demands was 15%, which is considered acceptable for practical design purposes.

A low computational cost method for vibration analysis of rectangular plates subjected to moving sprung masses

Nikkhoo, Ali,Asili, Soheil,Sadigh, Shabnam,Hajirasouliha, Iman,Karegar, Hossein Techno-Press 2019 Advances in computational design Vol.4 No.3

A low computational cost semi-analytical method is developed, based on eigenfunction expansion, to study the vibration of rectangular plates subjected to a series of moving sprung masses, representing a bridge deck under multiple vehicle or train moving loads. The dynamic effects of the suspension system are taken into account by using flexible connections between the moving masses and the base structure. The accuracy of the proposed method in predicting the dynamic response of a rectangular plate subjected to a series of moving sprung masses is demonstrated compared to the conventional rigid moving mass models. It is shown that the proposed method can considerably improve the computational efficiency of the conventional methods by eliminating a large number of time-varying components in the coupled Ordinary Differential Equations (ODEs) matrices. The dynamic behaviour of the system is then investigated by performing a comprehensive parametric study on the Dynamic Amplification Factor (DAF) of the moving loads using different design parameters. The results indicate that ignoring the flexibility of the suspension system in both moving force and moving mass models may lead to substantially underestimated DAF predictions and therefore unsafe design solutions. This highlights the significance of taking into account the stiffness of the suspension system for accurate estimation of the plate maximum dynamic response in practical applications.

Seismic performance of CFS shear wall systems filled with polystyrene lightweight concrete: Experimental investigation and design methodology

Mohammad Rezaeian Pakizeh,Hossein Parastesh,Iman Hajirasouliha,Farhang Farahbod 국제구조공학회 2023 Steel and Composite Structures, An International J Vol.46 No.4

Using light weight concrete as infill material in conventional cold-formed steel (CFS) shear wall systems can considerably increase their load bearing capacity, ductility, integrity and fire resistance. The compressive strength of the filler concrete is a key factor affecting the structural behaviour of the composite wall systems, and therefore, achieving maximum compressive strength in lightweight concrete while maintaining its lightweight properties is of significant importance. In this study a new type of optimum polystyrene lightweight concrete (OPLC) with high compressive strength is developed for infill material in composite CFS shear wall systems. To study the seismic behaviour of the OPLC-filled CFS shear wall systems, two full scale wall specimens are tested under cyclic loading condition. The effects of OPLC on load-bearing capacity, failure mode, ductility, energy dissipation capacity, and stiffness degradation of the walls are investigated. It is shown that the use of OPLC as infill in CFS shear walls can considerably improve their seismic performance by: (i) preventing the premature buckling of the stud members, and (ii) changing the dominant failure mode from brittle to ductile thanks to the bond-slip behaviour between OPLC and CFS studs. It is also shown that the design equations proposed by EC8 and ACI 318-14 standards overestimate the shear force capacity of OPLC-filled CFS shear wall systems by up to 80%. This shows it is necessary to propose methods with higher efficiency to predict the capacity of these systems for practical applications.

Cyclic behavior of cold formed steel frames in-filled with styrene concrete

Mohammad Rezaeian Pakizeh,Hossein Parastesh,Farhang Farahbod,Iman Hajirasouliha 국제구조공학회 2021 Steel and Composite Structures, An International J Vol.41 No.3

Light Steel Frame (LSF) systems are increasingly used as sustainable design solutions in modern construction. Using light weight concrete as infill material in LSF systems offers several advantages such as increased integrity, strength, ductility and fire resistance, while it also prevents premature local buckling failure modes. This research investigates the application of Styrene Concrete (SC) as light weight infill materials in LSF panels. Five full-scale LSF walls are examined to study the efficiency of using SC light weight infill material in improving the cyclic behavior of LSF panels. The specimens are designed to assess the effects of infill material as well as using strap bracing, hobnail and hole on the studs. The key seismic performance parameters including failure mode, load-bearing capacity, lateral stiffness, ductility, stiffness deterioration and energy dissipation capacity are obtained for each case. The experimental results demonstrate that the application of non-structural lightweight concrete as infill material in LSF shear walls has significant positive effects on their seismic performance by postponing the buckling of the steel frame members and changing the dominant brittle failure to a ductile failure mode. The interaction between LSF members and SC infill material could also considerably improve the lateral performance of the frame system. It is shown that adding the hobnails to the vertical studs increased the lateral stiffness and resistance of the frames by 45% and 28%, respectively. While the presence of a hole in the studs had little effect on the lateral resistance of the wall, it increased the energy dissipation capacity and ductility of the system by up to 18% and 6%, respectively.